Seasonally recurring water column hypoxia impacts the partitioning of organic carbon mineralization and resultant manganese–iron–sulfur–phosphorus dynamics in coastal sediments

Abstract To elucidate the impact of changing water column hypoxia (WCH), defined as dissolved oxygen concentration < 63 μ M, on sediment biogeochemical processes, we conducted a fine‐scale investigation on the partitioning of major organic carbon (C org ) mineralization processes, the reduction of manganese (MnR), iron (FeR) and sulfate (SR), and the resulting Mn–Fe–S–P dynamics at two geochemically contrasting coastal sites with different manganese content. At the manganese‐rich site with moderate WCH conditions, MnR and FeR dominated C org mineralization in surface sediments, comprising up to 87% (0–1 cm depth) and 88% (1–2 cm depth) of C org mineralization, respectively. In particular, the reoxidation of Fe 2+ to Fe‐oxides coupled to abiotic MnR in the surface sediments induced P adsorption to Fe(Mn)‐oxides, thereby suppressing benthic P release. In contrast, at the relatively organic‐rich and sulfidic site with severe WCH, SR comprised up to 87% of the C org mineralization, and enhanced HS − production stimulated benthic P release. Moreover, the impact of WCH on the anoxic sediment condition persisted longer even after the termination of WCH, implying that the spatial–temporal expansion of WCH ultimately grants sulfidic sediments a role as a chronic source of P in most coastal ecosystems. Our results, combining fine‐scale C org mineralization rate measurements with comprehensive geochemical inventory analysis, provide crucial information to elucidate cause‐effect relationships behind observed phenomena on the distributions and flux of elements in coastal environments where WCH is expanding due to climate change and excessive human activities.